In the discussion of the prior art, reference will be made in particular to a DML, which is one type of direct modulation resonant cavity device and which is exemplary of direct modulation resonant cavity devices in general.
The output of a DML must be modulated in order to generate an output signal that is capable of transmitting data over an optical communications medium. The modulation, which involves varying the intensity of the optical output, can be either digital or analogue. In the digital case, data is transmitted in the form of symbols, which are represented by different states of the optical output. In the simplest case, a symbol represents a single bit, in which case the optical output changes from a high to low state to represent logical 1's and 0's.
The output of a laser is commonly evaluated by means of an “eye diagram”, which is created by taking the time domain laser output level and overlapping the traces on an oscilloscope for a certain number of symbols using the data rate to trigger the horizontal sweep. From the eye diagram, properties such as overshoot, wherein the output level exceeds the nominal high level, can be determined. The overshoot is due to the dynamics of the laser being sub-critically damped and is responsible for “eye ringing”, which may cause the shape of the output “eye” pattern to be modified such that it does not fit within the “template” of acceptable performance used to ensure that the signal is capable of being decoded after transmission along an optical fibre.
At high bit rates, for example 10 Gbit/s, the quality of an optical waveform sent down the optical fiber is degraded because of ringing on the logical 1's and the logical 0's. A major issue is overshoot on the rising edge during the transition to the logical 1 level. The optical waveform generated by the laser has to meet very stringent specifications. For telecom links part of the specification is described by a mask (for example the SONET mask) which the waveform should not encroach on. A schematic example of a failed mask is shown in FIG. 1. Overshoot in the logical 1 state results in the waveform encroaching on the mask. This is a long-standing problem that significantly restricts the usefulness of a DML, particularly for applications involving high speed digital or analogue modulation. Amplitude modulation (both digital and analog) is advantageous over alternative frequency or phase modulation schemes in that it is possible to avoid the need for a separate modulator. The amplitude of the light generated by the laser can be modulated directly by an electrical drive signal.
Ringing happens for all lasers, but on a different time scale depending on the type of lasers. Damped periodic oscillations are caused by an intrinsic resonance in the non-linear laser system. In operation a laser is biased above the lasing threshold by being driven to create a population inversion of electrical charge carriers. When the drive current increases, for example in the transition from a 0 to a 1 optical output of a binary system, the degree of electrical charge carrier inversion increases. This increase in the degree of electrical charge carrier inversion is accompanied by an increase in the optical gain in the optical cavity of the laser to such a point that lasing occurs and the light intensity in the cavity increases rapidly to a high level. The high light intensity depletes the carrier density through stimulated emission, which in turn decreases the optical gain and thus the light intensity. The reduced light intensity will then allow the electrical carrier density to increase again, thus commencing another cycle of the optical power within the laser. These cycles that occur when there is a change in the drive current are damped, and the length of time that the laser takes to stabilise depends on factors such as the driving conditions and the laser design. At 10 Gb/s and above it is extremely difficult to remove all the cycles, and often the first cycle remains. This appears as an overshoot on the rising edge of the eye pattern. At lower frequencies, such as 2.5 Gb/s and below it is much easier to avoid the overshoot because the laser may be perturbed more slowly. The speed of the rising edge of the electrical drive signal is critical.
Damping is not an easy property to modify by design. Damping is increased by a larger photon density inside the optical cavity, by a larger gain saturation or by longer carrier transport time. Unfortunately these parameters cannot be readily modified to improve the laser performance.
To improve the photon density significantly the laser has to be driven harder (with larger drive currents), which tends to decrease the extinction ratio between the logical 1 and the logical 0 states (typically 10 dB). This degrades system performance. It is also possible to modify the cavity coatings or grating strength (in the case of a DFB laser), but the increase in the photon density is usually small, and in the case of a larger grating strength can lead to other problems such as a decrease in the single-mode yield.
It has been suggested that gain saturation can be modified slightly by choosing suitable materials in the optically active region of the waveguide supporting the propagation of the optical mode in the diode laser. Unfortunately, reported variations in gain saturation are small and are not easily translatable into different material systems or different operating wavelengths since the fundamental physical mechanism behind gain saturation in commercial diode lasers is still not fully understood.
There is some evidence that carrier transport time can be increased in diode lasers by thickening the optically active region of the waveguide core. However, in commercial diode lasers comprising multiple quantum well (MQW) active layers the understanding of how much the carrier transport time can be varied in this way is still incomplete. Moreover, simulations indicate that too long a transport time leads to signal patterning or inter-symbol interference (ISI) and a large increase in chirp, or frequency excursion. This also strongly limits performance at higher bit rates, such as 20 Gbit/s or 40 Gbit/s, and longer system reaches, such as greater than 20 km at 10 Gbit/s.
The possibility of modifying the laser drive current waveform to improve the optical signal has been investigated. In an example a pre-pulse is used before the 0 to 1 transition in the current waveform. However, such an approach requires very fast and special laser drivers in order to realize such waveforms. Moreover different drivers are needed for different lasers, or the driver would be required to have some tuneable characteristics. That means that the laser driver cost would probably be too prohibitive to use. Examples of such an approach can be found in: L. Illing and M. B. Kennel, IEEE Journal of Quantum Electronics, vol. 40, no. 5, p. 445-452 (2004); and N. Dokhane and G. L. Lippi, IEE Proceedings-Optoelectronics, vol. 151, no. 2, p. 61-68 (2004).
A different approach consisting of injecting light into the directly-modulated laser from a high power continuous wave (CW) diode laser with a wavelength set at transparency (i.e. the wavelength at which there is neither gain nor loss) was also demonstrated in: G. Morthier and B. Mocyersoon, IEEE Photonics Technology Letters, vol. 16, no. 7, p. 1616-1618 (2004). This approach is not very attractive because it complicates significantly the packaging of the laser, requiring an optical alignment between the two laser chips. It also significantly increases the power dissipation by the device and would be difficult to use for uncooled operation.
In order to explain the significance of the prior art, it will first be useful to introduce the concept of chirp. Chirp is a change in frequency of the light output as a laser is modulated, which results from differences in the laser optical cavity length under different drive conditions, and at the point when the drive condition changes. Three different types of chirp can be considered.
Adiabatic chirp is a stable difference in the lasing frequency for a 1 compared with a 0, arising from a difference in equilibrium electrical carrier density between the two modulation states. This type of chirp exists and is invariant over modulation frequencies ranging from DC up to >100 Gb/s.
Thermal chirp corresponds to temperature induced frequency changes resulting from fluctuations in power dissipation in the laser as the device is modulated. This type of chirp is most significant at low modulation frequencies and typically will not be observed above 100 Mb/s.
Transient chirp is an unstable fluctuation in the lasing frequency caused by a resonant oscillation between the electrical carrier density and the optical photon density which occurs when the carrier injection and/or the optical power within the laser cavity change. This type of chirp is dominant at modulation rates close to the laser relaxation oscillation frequency which, depending on the design of laser, can vary between 1 and 20 GHz. For a very fast switching of the electrical drive signal (typically at higher modulation rates), this can be the dominant effect in bandwidth broadening.
U.S. Pat. No. 4,669,086 to Kaede teaches of cancelling the chirp, or instantaneous frequency deviation, induced when directly modulating a laser at moderately low frequencies. In this method a control signal, which is a phase-shifted and attenuated version of the modulation signal, is applied to a separate electrode on a separate section of the laser cavity. This section is biased below (the lasing) threshold and because of this the carrier density, and thus the refractive index, is strongly modulated by the control signal. Above threshold the carrier density is approximately “pinned” and varies less with modulation current. The choice of phase shift between control and modulation signals is such that the index change in the control section is in antiphase to the index change generated in the main modulation section of the laser. If the product of the index change and length for the two sections are properly balanced (equal and opposite) then the net optical frequency of the laser under modulation will be stable. However, this method is only suitable for low frequency modulation because as the modulation frequency exceeds the differential carrier lifetime of the laser the modulation response of the carrier density rapidly reduces. The differential carrier lifetime of a typical directly modulated laser at biases below threshold would tend to be in the 1-2 ns range and consequently this approach would be limited to modulation frequencies less than around 0.5-1 GHz. In addition, the time constants associated with carrier transport and carrier recombination mechanisms produce a modulation frequency dependent phase relationship between the control current and the carrier density in the control section. This will limit this approach to narrow band analogue modulation or low data rate digital modulation applications.
Kaede is concerned with optically modulating the laser output with a drive signal that comprises an amplitude modulated electrical carrier signal, from driver 301, and shown in Kaede FIG. 3a. In such prior art a high frequency substantially sinusoidal electrical carrier is amplitude modulated with a data signal at a much lower data rate. Each bit corresponding to many cycles of the carrier. This difference in data rate and carrier frequency is necessary, so that the bandwidth of the data signal is less than the carrier frequency (otherwise data is lost for example).
When the optical signal is detected the output electrical signal is again an amplitude modulated (AM) carrier, with the data being encoded in the amplitude. Consequently the shapes of the individual periods of the carrier are relatively unimportant, so long as the amplitude relates to that of the data signal originally used to encode the carrier. Thus Kaede cannot be said to teach a method of improving the quality of amplitude modulation that is relatively unimportant to the device of Kaede, and in particular Kaede does not address the phenomenon of overshoot.
U.S. Pat. No. 5,502,741 to Carroll teaches a method of achieving independent control of amplitude or frequency modulation, which are undesirably interrelated in a conventional, single section DFB laser (or Fabry-Perot laser—i.e. a laser with no grating). This patent describes a two section top electrode that is driven in a “push-pull” drive configuration, in which a second section is driven by the same signal waveform as a first section, after having been inverted and DC shifted. This approach decreases the chirp (rate of wavelength change), but its impact on damping is unknown. This approach is difficult to implement because the necessary signal driver does not presently exist as an off-the-shelf product, and would be prohibitively expensive as a custom-made product.
Carroll is concerned with the use of an anti-phase modulation signal driving scheme. Carroll discloses a design in which both parts of the device lase, although each section may not necessarily lase continuously. Further, the intention of Carroll is also to control the total photon population, and maintain it constant: “the anti-symmetric modulation of current operates in a push-pull mode to keep substantially constant the total photon population within the laser”. Carroll teaches that the laser should be substantially longitudinally symmetric (along the waveguide).
Another set of designs of multiple section lasers is known where both sections are driven in DC, for tunable lasers or to generate microwave signals for examples. Yet another set of designs involves the so-called “gain-lever” effect, in which one section is DC biased and another section is modulated.